The present invention relates to electronic devices, and, more particularly, to semiconductor devices useful in connection with memories.
Typical semiconductor random access memories available for storage of information, as in a computer system, are volatile in the sense that an interruption of power will lead to a loss of contents. Both dynamic RAMs (DRAMs) and static RAMs (SRAMs) require a constant power supply, and various backup power supply arrangements have been proposed. For example, Dallas Semiconductor Corporation manufactures a CMOS SRAM which is combined with its own backup battery in a standard size package pinout to achieve a nonvolatile memory. Dallas Semiconductor Corporation also manufactures alternative battery backup devices such as the DS1210 memory controller which fits between a microprocessor and a standard SRAM and also connects to a backup battery. The DS1210 senses power supply voltage, which typically is a nominal 5 volts, and if this voltage drops below battery level voltage, typically a nominal 3 volts, the DS1210 switches the SRAM from the power supply to battery power. Also, the DS1210 may connect to two batteries and select the higher voltage battery for backup operation plus switch from the first backup battery to the second during backup operation if the voltage of the first battery drops much below that of the second battery.
Another aspect of a memory controller invokes write protection of the controlled memory as the voltage of the power supply falls. Such write protection helps avoid corruption of data in the memory.
A problem with the existing controllers is power consumption; all battery backup devices should draw as little current as possible during backup operation in order to prolong battery life.
Another problem with battery backup operation relates to the environmental concerns of disposal of used batteries. In particular, a 3 volt battery backing up a 5 volts power supply would typically be considered used up when its voltage drops to about 2 volts, and the battery at 2 volts presents an environmental hazard. Thus this would require an additional procedure of discharging the battery after removal from a circuit board and prior to disposal.
The present invention provides a memory controller for memories with programmable write protection for blocks of a controlled memory in addition to write protection upon falling power supply voltage, with low current draw switching between batteries during backup operation by use of MOSFET switches, with a supravoltage-activated sleep mode which isolates connected batteries to avoid leakage current discharge during storage, and with a battery discharge circuit to intentionally fully discharge batteries as may be required by environmental concerns.